1. Field of the Invention
The present invention relates to an ink supply device for an inkjet printer, and more particularly, to an ink supply device for an inkjet printer which provides a stable ink supply by accurately controlling pressure loss in an ink supply channel for supplying ink to an inkjet head, through controlling the temperature and pressure of the ink.
2. Description of the Related Art
Conventionally, an inkjet printer is known as an image recording apparatus, which comprises an inkjet head having an arrangement of a plurality of nozzles and which records images on a recording medium by discharging ink from the nozzles toward the recording medium while causing the inkjet head and the recording medium to move relatively to each other.
In an inkjet head of this kind, ink is supplied to pressure chambers from an ink tank, via an ink supply channel, and by deforming piezoelectric elements by supplying electrical signals corresponding to the image data to the piezoelectric elements, the diaphragms constituting a portion of each pressure chamber are caused to deform, thereby reducing the volume of the pressure chamber and causing the ink inside the pressure chamber to be discharged from a nozzle in the form of a droplet.
In this case, when supplying ink from an ink tank storing ink to the pressure chambers, via an ink supply channel, pressure loss occurs in the ink at the filter provided in the ink supply channel to remove foreign matter from the ink, and at the joint sections or elbows of the ink supply channel. If the ink pressure is reduced significantly, then there is a risk that ink may not be discharged suitably from the nozzles.
In particular, if high-viscosity ink is discharged continuously at a high printing duty, then the viscous resistance of the ink supply channel increases in regions such as the filter section, and large pressure loss is caused by this viscous resistance. Therefore, the internal pressure of the head is reduced, the size of the discharged ink droplets decreases, and the density of the recorded image declines. In the worst case scenario, a discharge error occurs.
Therefore, it is necessary to provide an ink supply having a stable pressure, and in the prior art, various proposals have been made in order to achieve stable ink supply and stable ink discharge.
For example, technology is known in which a temperature adjustment device is provided which adjusts the temperature partially in a prescribed portion of the ink supply channel only, such as the filter member provided in the ink supply channel, in order to maintain uniform ink discharge volume and discharge frequency (see, for example, Japanese Patent Application Publication No. 8-156280). In this way, it is possible to stabilize the negative pressure inside the ink tank in accordance with the ink characteristics, and to reduce the viscous resistance of the ink in the ink supply channel (and especially in the filter section) during high-duty printing, thereby preventing increased pressure loss in the filter section due to increased ink viscous resistance. Therefore, the discharge characteristics can be stabilized, even when operating at high duty.
Furthermore, technology is also known in which a heating device capable of heating the region of the filter is interposed in the ink channel, thereby heating the ink passing through the filter and reducing the viscosity of the ink (see, for example, Japanese Patent Application Publication No. 3-295661). Consequently, the fluid resistance of the ink when passing through the filter is reduced, thereby preventing blockages even if the ink discharge ports are extremely fine, while also increasing the speed of ink replenishment to the ink discharge ports.
Moreover, technology is also known which comprises: a recording head which discharges high-viscosity ink that has been reduced to low viscosity by raising the temperature thereof, the ink being discharged while it is in a low-viscosity state, a temperature determination device which determines the temperature of the high-viscosity ink, a supply channel heating device which heats the high-viscosity ink in the supply channel, and a heating control device which controls the supply channel heating device on the basis of the temperature determination device. Furthermore, the heating control device controls a tank heating device for heating the high-viscosity ink in the ink tank and a head heating device for heating the high-viscosity ink in the recording head in such a manner that the heating temperature of at least one of the tank heating device or the head heating device is high, thereby heating the high-viscosity ink inside the supply channel, reducing the ink to a low viscosity, and hence enabling the ink to be discharged in a stable fashion (see, for example, Japanese Patent Application Publication No. 2003-127417).
However, in the device described in Japanese Patent Application Publication No. 8-156280, the heating temperature is changed according to the printing duty in order to reduce power consumption, and the ink temperature is adjusted in three stages with respect to printing duty. Nevertheless, when printing with high-viscosity ink, it is necessary to control the ink temperature in a stepless fashion in accordance with the printing duty, but in the case of the three-stage control disclosed in the patent reference, it is not possible to achieve suitable control when discharging high-viscosity ink. Moreover, there is a further problem in terms of the responsiveness of temperature control, when the printing duty changes.
Moreover, Japanese Patent Application Publication No. 3-295661 discloses technology for reducing pressure loss by reducing the ink viscosity through heating the filter section, or the like. However, if the heating of the filter section is controlled in accordance with the head discharge frequency, in real time, it is not possible to adjust the ink temperature accurately, due to delay in the temperature control, and a problem arises in that a heater cannot be disposed on the upstream side of the filter section of the ink supply channel, for instance. Therefore, the ink temperature cannot be controlled effectively in an accurate manner.
Furthermore, in the device described in Japanese Patent Application Publication No. 2003-127417, if the ink temperature in the pressure loss region of an ink supply section is set to a higher temperature than the internal temperature of the head, which guarantees ink discharge properties (in other words, if the temperature of the pressure loss region>internal temperature of head), in order to reduce the viscous resistance in the ink supply section, then a problem arises in that the temperature inside the head cannot be controlled.